1. Field of the Invention
The present invention relates to a temperature controlling method and apparatus for a thermal printhead useful for thermal recording of an image on reversible thermosensitive recording materials or normal thermosensitive recording materials, and more particularly to a method and apparatus for controlling a temperature of a thermal printhead to a temperature suitable for erasing and recording an image under various temperature conditions.
The present invention is based on Japanese Patent Application No. 07-315882 filed on Nov. 10, 1995 incorporated herein by reference.
2. Discussion of Background
A reversible thermosensitive recording material is well known and has wide-scale utilization not only as a recording material for facsimile machines, computers and word processors, but also as a material for cards, such as a prepaid card on which an image is recorded on a predetermined area. In particular, since a reversible thermosensitive recording material can have image information repeatedly erased from it and printed on it, cards utilizing such a reversible thermosensitive recording material can be used repeatedly.
A reversible thermosensitive recording material is disclosed in, for example, Japanese Laid-Open Patent Application No. 4-44887. Such a reversible thermosensitive recording material reversibly changes transparency upon application of proper heat thereto and accordingly can be used to repeatedly print and erase image information on it. As shown in FIG. 1, a reversible thermosensitive recording material 1 has a substrate 41 such as polyester film, a recording layer 44 which is formed on the substrate 41, and a recording material including a complex of a polymer 42 and a low-molecular-weight organic compound 43. Particles of the low-molecular-weight organic compound 43 are dispersed in the polymer 42, and a protective layer 45 is formed on the recording layer 44. As shown in FIG. 2, transparency of the recording material of the recording layer 44 changes when the recording material is heated or cooled. For example, when the recording material in an opaque state below room temperature T.sub.1 (i.e., state A) is heated, transparency of the recording material begins to increase at a temperature of T.sub.2 and reaches a maximum transparent state B at a temperature T.sub.3. If the recording material in a maximum transparent state B is cooled to room temperature (i.e., to state C), transparency of the recording material is maintained. If the recording material which is in the maximum transparent state C is heated to a temperature T.sub.4, higher than the temperature T.sub.3 the recording material reaches an intermediate state (i.e., state D) between the maximum transparent state B and the maximum opaque state A, as shown by a dotted line CD in FIG. 2. As the recording material in the intermediate state D is cooled to below room temperature T.sub.1, the recording material returns to the maximum opaque state (i.e., state A), as shown by a dotted line DA in FIG. 2. The reversible thermosensitive recording material 1 has a property of reversibly changing transparency when a particle of the low-molecular-weight organic compound 43 changes between a single crystal state and a polycrystalline state upon application of heat. When a particle of the low-molecular-weight organic compound 43 in the recording material 1 is in the single crystal state, the particle transmits light and accordingly the recording material 1 appears transparent. On the other hand, when the particle is in the polycrystalline state, the particle scatters light and accordingly the recording material appears opaque. By using this property of the particles of the low-molecular-weight organic compound 43, the recording material 1 can be used to repeatedly record and erase image information.
Japanese Laid-Open Patent Application No. 5-4446 discloses an apparatus which repeatedly records and erases image information on the reversible thermosensitive recording material 1. As shown in FIG. 3, the apparatus has an image erasing thermal printhead 6 and an image recording thermal printhead 5. When the reversible thermosensitive recording material 1, fed by a feeding roller 3 and platen rollers 4, is detected by a position detecting sensor 46 which is placed before the image erasing thermal printhead 6, the image erasing thermal printhead is heated and kept at a temperature which makes the reversible thermosensitive recording material 1 transparent, thus erasing previously recorded opaque image information. Then the image recording thermal printhead 5 records new opaque image information on the transparent recording layer of the reversible thermosensitive recording material 1. By repeating the above operations, the reversible thermosensitive recording material 1 can be repeatedly used.
However, when a thermosensitive recording material is used for a card, such a prepaid card, often times a card which has been exposed to the outside air is inserted into a recording device having a thermal printhead for recording or erasing. In such a case, a temperature of the card, i.e., a temperature of a thermosensitive recording material in the card, may happen to be below 0.degree. C. or conversely more than 50.degree. C. In the above cases, if a constant heat quantity suitable for the recording or erasing of image information on the thermosensitive recording materials exposed to a normal temperature is applied to a card having a temperature which is below 0.degree. C. or above 50.degree. C., problems such as unsatisfactory image erasing or image recording tend to occur because the heat quantity is not suitable to erase or record image information on such a card.
In particular, when previously recorded image information on a recording layer of a reversible thermosensitive recording material is erased and then new image information is again recorded on the recording layer, or when an image having several different degrees of transparency is recorded on a recording layer of a reversible thermosensitive recording material, heat that is applied to the recording layer of the reversible thermosensitive recording material has to be strictly controlled to perform image erasing and recording. If heat applied to the recording layer in the reversible thermosensitive recording material to erase or record image information is not strictly controlled, the recording quality of a recorded image is degraded. In addition, when an image is recorded on a thin card such as a prepaid card or a thick card such as a credit card with the above image erasing and recording apparatus, the recording quality of recorded images on the thick card tends to be degraded, particularly in relatively low temperature environments because of insufficient transfer of heat for image recording or image erasing due to the thickness of the card.
Because of the above reasons, a need exists for a thermal recording method and an apparatus which can erase previously recorded image information and/or record image information on a reversible thermosensitive recording material regardless of a temperature of the recording material before the image erasing or recording operation.